linux-kernel - Re: [PATCH v4 4/4] KVM: SVM: Add Bus Lock Detect support

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Message-ID: <5fbd65ae-40a2-4434-af44-84c749e5fdd2@amd.com>
Date: Fri, 14 Nov 2025 14:20:14 +0530
From: Shivansh Dhiman <shivansh.dhiman@....com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Ravi Bangoria <ravi.bangoria@....com>, <tglx@...utronix.de>,
	<mingo@...hat.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com>,
	<pbonzini@...hat.com>, <thomas.lendacky@....com>, <jmattson@...gle.com>,
	<hpa@...or.com>, <rmk+kernel@...linux.org.uk>, <peterz@...radead.org>,
	<james.morse@....com>, <lukas.bulwahn@...il.com>, <arjan@...ux.intel.com>,
	<j.granados@...sung.com>, <sibs@...natelecom.cn>, <nik.borisov@...e.com>,
	<michael.roth@....com>, <nikunj.dadhania@....com>, <babu.moger@....com>,
	<x86@...nel.org>, <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<santosh.shukla@....com>, <ananth.narayan@....com>, <sandipan.das@....com>,
	<manali.shukla@....com>, <yosry.ahmed@...ux.dev>
Subject: Re: [PATCH v4 4/4] KVM: SVM: Add Bus Lock Detect support

On 11-11-2025 20:45, Sean Christopherson wrote:
> On Tue, Nov 11, 2025, Shivansh Dhiman wrote:
>> On 17-08-2024 05:43, Sean Christopherson wrote:
>>> On Thu, Aug 08, 2024, Ravi Bangoria wrote:

...

> 
>> ===========================================================
>> Issue 1: Interception still enabled after enabling LBRV
>> ===========================================================
>> Using the 6.16 upstream kernel (unpatched) I ran the KUT tests and they passed
>> when run from both the bare metal and from inside a L1 guest. However for L2
>> guest, when looking at the logs I found that RDMSR interception of LBR MSRs is
>> still enabled despite the LBRV is enabled for the L2 guest. Effectively, the
>> reads are emulated instead of being virtualized, which is not the intended
>> behaviour. KUT cannot distinguish between emulated and virtualized RDMSR, and
>> hence the test passes regardless.
> 
> I haven't looked closely at your patch or at Yosry's patches, but I suspect this
> was _just_ fixed:
> 
> https://lore.kernel.org/all/20251108004524.1600006-1-yosry.ahmed@linux.dev

Thanks Sean. I tested Yosry's patches and they indeed have solved this issue.

> 
>> ===========================================================
>> Issue 2: Basic LBR KUT fails with Sean's implementation
>> ===========================================================
>> After using your implementation, all KUTs passed on the bare metal. With LBRV
>> enabled for L2, RDMSR interception of LBR MSRs is disabled as intended.
>> However, when running KUT tests inside an L1 guest, the tests fail.
> 
> Same story here: I haven't had cycles to actually look at code, but Yosry also
> posted a pile of changes for KUT:
> 
> https://lore.kernel.org/all/20251110232642.633672-1-yosry.ahmed@linux.dev

This issue was also related to buggy LBRV in the kernel. Yosry's patches have
fixed this issue as well and I've verified it. There was a slight flakiness in
the KUT which was later fixed by another patch by Yosry's. [1]

Thanks,
Shivansh

1. https://lore.kernel.org/all/20251113224639.2916783-1-yosry.ahmed@linux.dev/